1. Field of the Invention
The present invention relates to a method of driving an ink-jet printhead. More particularly, the present invention relates to a method of driving an ink-jet printhead using a driving waveform capable of representing gradation.
2. Description of the Related Art
In general, ink-jet printheads eject fine droplets of ink for printing at desired positions on a recording medium to print an image of a predetermined color. Ink-jet printheads may be classified into two types according to a mechanism used to eject an ink droplet. A first type is a bubble jet type ink-jet printhead, which generates a bubble in ink using a heat source to eject an ink droplet by an expansion force of the bubble. A second type is a piezoelectric type ink-jet printhead, which ejects an ink droplet by pressure applied to ink due to a deformation of a piezoelectric body.
FIG. 1 illustrates a structure of a conventional piezoelectric type ink-jet printhead.
Referring to FIG. 1, an ink-jet printhead 10 includes a pressure chamber 15 filled with ink to be ejected. Ink supply paths 28 and 34, through which ink is supplied from an ink reservoir 35 to a pressure chamber 15, are connected to one side of the pressure chamber 15. Ink discharge paths 29 and 36 are connected to the other side of the pressure chamber 15. A nozzle 13 for ejecting the ink is formed at an end portion of the ink discharge paths 29 and 36. A vibration plate 23 is provided in an upper portion of the pressure chamber 15. A piezoelectric actuator 25 for providing a driving force to eject the ink by vibrating the vibration plate 23, which changes a volume of the pressure chamber 15, is provided on the vibration plate 23. The piezoelectric actuator 25 includes a common electrode 26 formed on the vibration plate 23, a piezoelectric film 14 formed of a piezoelectric material on the common electrode 26, and a driving electrode 27 formed on the piezoelectric film 14 for applying a driving voltage to the piezoelectric film 14.
In such a piezoelectric type ink-jet printhead 10, when a driving pulse having a predetermined driving voltage is applied to the piezoelectric film 14 through the driving electrode 27, the vibration plate 23 is bent by the deformation of the piezoelectric film 14, thereby decreasing the volume of the pressure chamber 15. As the volume of the pressure chamber 15 decreases, the pressure in the pressure chamber 15 increases. This increase in pressure in the pressure chamber 15 causes the ink in the pressure chamber 15 to be ejected out of the printhead 10 through the nozzle 13. Then, when the driving pulse applied to the piezoelectric film 14 is removed, the vibration plate 23 is restored to an original shape thereof and the volume of the pressure chamber 15 increases. As the volume in the pressure chamber 15 increases, the pressure in the pressure chamber 15 decreases. This decrease in pressure causes ink to be absorbed from the ink reservoir 35 through the ink supply paths 34 and 28, thereby refilling the pressure chamber 15 with ink.
The above-described piezoelectric type ink-jet printhead is advantageous in representing gradation because it can eject ink droplets having a variety of volumes through the nozzle 13, which has a uniform diameter, depending on the waveform of the driving pulse applied to the piezoelectric actuator 25.
FIG. 2 illustrates driving waveforms for use in a conventional method of driving the ink-jet printhead shown in FIG. 1.
The driving pulses shown in FIG. 2 have waveforms to adjust a volume of a droplet in two steps. More specifically, a first driving pulse to eject a droplet having a relatively smaller volume includes a first pulse and a second pulse. A second driving pulse to eject a droplet having a relatively larger volume includes only a second pulse. The second pulse is a main pulse providing a driving force sufficient to eject an ink droplet, while the first pulse is an auxiliary pulse that is not sufficient to cause ejection of an ink droplet.
When the first pulse is initially applied to the piezoelectric actuator 25, prior to application of the second pulse, the vibration plate 23 vibrates slightly due to the first pulse before the droplet is ejected and the meniscus of the ink in the nozzle 13 retreats. When the second pulse for ejecting the droplet is applied at the point when the meniscus of the ink retreats, the volume of the droplet is reduced. Accordingly, a diameter of a dot printed on the recording medium decreases. When the second driving pulse having only the second pulse is applied to the piezoelectric actuator 25, a droplet having a relatively larger volume is ejected. Accordingly, the diameter of a dot printed on the recording medium increases.
However, in the above driving method, accurately adjusting a timing of the retreat of the meniscus of the ink is difficult. The speed when a smaller droplet is ejected is slower than that when a larger droplet is ejected. Accordingly, a position of the dot on the recording medium is changed, which deteriorates print quality.
FIG. 3 illustrates driving waveforms used in another conventional method of driving an ink-jet printhead.
According to the driving waveforms shown in FIG. 3, by selectively applying a first pulse to eject a droplet having a small volume and a second pulse to eject a droplet having a large volume, droplets having three different volumes can be ejected. More specifically, when a first driving pulse including only the first pulse is applied to the piezoelectric actuator 25, a droplet having a small volume is ejected and a dot having a small diameter is printed on the recording medium. Although not shown, when only the second pulse is applied to the piezoelectric actuator 25, a droplet having a large volume is ejected and a dot having a large diameter is printed on the recording medium. When a second driving pulse including both the first and second pulses is applied to the piezoelectric actuator 25, a droplet having a small volume is initially ejected and a droplet having a large volume is ejected to overlap the droplet having the small volume, which prints a dot having the largest diameter on the recording medium.
According to the above driving method, although there is a difference in the ejection speed of the droplet having a relatively larger volume and that of the droplet having a relatively smaller volume, since the slow speed of the droplet having a smaller volume can be compensated for by applying the first pulse to eject the smaller droplet prior to the second pulse to eject the larger droplet, the two droplets can be located at the same position on the recording medium.
However, in the above conventional driving method, ejection timing control is difficult with respect to two droplets having different ejection speeds. Furthermore, when two droplets are overlapped to print a dot having the largest diameter on the recording medium, it is difficult for the printed dot to have a perfect circle and the diameter of the dot is not proportional to the volumes of the ejected droplet.